I see three basic options:
1. Add ONE little checkbox called "Fix Width" (or whatever). If ON, then the width of the graph is fixed to the width of the window. The height of the graph changes as appropriate to maintain the stated proportion. [note: doing Height rather than Width doesn't make a lot of sense in most use cases]
2. Add TWO little checkboxes called "Fix Width" and "Fix Height" (or whatever). If ON, then the width and/or height of the graph is fixed to the width and/or height of the window.
3. Leave things as they are.
It seems to me that #1 is the right way to do it (other than #3 of course). Anyone have any opinions?